The invention is directed to roller assemblies for linearly transporting one or more pipe segments, or joints, such as pipelines, and in particular, to linear pipe recovery/lay tensioners for pulling a portion of pipeline from the ocean to be recovered on an offshore lay-barge and for laying pipeline from an offshore lay-barge and onto the ocean floor.
Linear pipe tensioners are generally known in the art. These devices are generally installed on a ship or lay barge to facilitate laying or retrieving pipe, such as pipelines and other pipe segments. Linear pipe tensioners are designed to maintain a constant tension on the pipe while the pipes are being lowered into, or retrieved from, the sea during offshore pipe laying operations.
Broadly, linear pipe tensioners consist of an upper and a lower track, each supported in a frame assembly. Pneumatically actuated squeeze bags within the track loops apply squeeze forces on the tracks holding the pipe. The tracks are driven by hydraulic motors through cage-mounted gear reducers. The upper track can be adjustable vertically for handling of various sizes of pipe. An electronics package comprising a control console and PLC operates the electronic components necessary for system operation. Pressure to track drive motors is controlled to maintain pipe tension within selected limits.
Two or more linear pipe tensioners can be arranged in series to provide the desired or necessary tension in the pipeline for laying or retrieving the pipeline.
In operation, generally, when an operator sets a tension value and starts the linear pipe tensioner, stepper motors mounted on the hydraulic pumps are rotated under command from the electronic control system. Greater rotation increases the volume of hydraulic fluid from the pumps to the motors. The motors then increase the amount of tension against the pipe.
Load cells, mounted between the linear pipe tensioners and their bedplates, transmit a signal proportional to the amount of tension against the pipe to the electronic control system. When the signal from the load cells balances the signal from the tension, a preset circuit the stepper motors stop rotating and hold their positions. This keeps the hydraulic pumps on a stroke which produces sufficient pressure to hold the desired tension. Small movements of the lay-barge do not produce any significant change in the tension. Movements beyond a pre-set dead-band will produce changes in tension transmitted back to the control system. The tension command will become unbalanced and the control system will drive the pumps producing pressure to the hydraulic motors to bring the tension back into line.